EIS and differential capacitance measurements onto single crystal faces in different solutions Part II: Cu(111) and Cu(100) in 0.

Transcription

1 Journal of Electroanalytical Chemistry 541 (2003) 13/21 EIS and differential capacitance measurements onto single crystal faces in different solutions Part II: Cu(111) and Cu(100) in 0.1 M NaOH V.D. Jović 1, *, B.M. Jović 2 Materials Engineering Department, Drexel University, Philadelphia, PA 19104, USA Received 26 June 2002; received in revised form 24 October 2002; accepted 7 November 2002 Abstract The electrochemical behavior of Cu(111) and Cu(100) in 0.1 M NaOH solution in the underpotential region of Cu 2 O formation has been investigated by cyclic voltammetry and EIS (differential capacitance) measurements. By the analysis of differential capacitance versus potential and differential capacitance versus frequency curves it is shown that the process of OH species adsorption/desorption, takes place on both faces of the copper substrate. It is also shown that, on Cu(111), slow reconstruction of the original surface occurs during the process of OH species adsorption/desorption, while on Cu(100), this process was found to be very fast involving a much greater amount of adsorbed OH species. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cu(111); Cu(100); OH adsorption; CPE; Differential capacitance; Surface reconstruction 1. Introduction The anodic oxidation of copper in alkaline solutions has been extensively studied in the past (a review of this work is given in the paper of Droog et al. [1]), although the initial stage of the oxidation process received little attention. Ambrose et al. [2] first observed a small anodic peak on the voltammogram, preceding the peak of Cu 2 O formation after careful examination at high sensitivity and high sweep rates. This peak has been tentatively assigned to the formation of soluble species of Cu(OH) 2 : Investigating the initial stages of anodic oxidation of poly crystalline [1] and single crystalline [3] copper surfaces in 1 M NaOH solution, Droog et al. [1,3] concluded that electrosorption of oxygen species * Corresponding author. Fax: / address: vdjovic@elab.tmf.bg.ac.yu (V.D. Jović). 1 On leave of absence from the Center for Multidisciplinary Studies University of Belgrade, P.O. Box 33, Belgrade, Yugoslavia. 2 On leave of absence from the Institute of Technical Sciences SASA, P.O. Box 745, Belgrade, Yugoslavia. occurs at potentials more negative than the potential of Cu 2 O formation. Härtinger and Doblhofer [4] investigated the electrochemical interface between Cu(111) and aqueous electrolytes containing fluoride and sulphate anions at various ph values. Differential capacitance measurements were characterized with well defined peaks at the potential of about /0.46 V versus SHE which was ascribed to the adsorption of OH species in all electrolytes investigated as a consequence of an electrosorption process of the following type: CuH 2 O l k ads k des Cu(OH) ads H e (1) The formation of a hydroxide adsorbate on a Cu(111) surface in 0.1 M NaOH at potentials negative with respect to the potential of Cu 2 O formation has been investigated by Härtinger et al. [5] using cyclic voltammetry and surface-enhanced Raman spectroscopy (SERS). Experiments were performed in deaerated solutions. The voltammograms presented were recorded on a relatively low sensitivity scale, showing only peaks of Cu 2 O and CuO formation and reduction. SERS spectra recorded at potentials negative with respect to /02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S ( 0 2 )

2 14 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 the potential of Cu 2 O formation exhibited a remarkable Raman band located around 700 cm 1. The intensity of this band was found to depend on potential as well as on time of holding the electrode at a given potential. It was concluded that this band corresponds to CuOH-surface species, which were formed upon cathodic reduction of oxidized Cu(111) electrode. The process leading to CuOH surf formation was formulated in terms of proton (water) reduction on an oxidized copper surface Cu(O) ad H e CuOH surf Cu(O) ad H 2 Oe CuOH surf OH (2a) (2b) assuming that the species Cu*(O) ad exists at potentials more negative than that for Cu 2 O reduction. It is interesting to note that after stepping the potential from /0.25 V versus SHE (just before the peak of Cu 2 O formation) to /0.85 V versus SHE, the intensity of the band at 700 cm 1 started to increase with time of holding the electrode at that potential, reaching its maximum after about 10 min. The same authors [5] also presented cyclic voltammetry and differential capacitance measurements on Cu(111) in a solution of 0.1 M NaOH. The differential capacitance (C diff ) was recorded at an ac frequency of v/62.83 Hz (f/10 Hz) and a sweep rate of 2.5 mv s 1. Two capacitance peaks were detected in the potential range between /1.2 V versus SCE and /0.5 V versus SCE with the shapes of the C diff versus E curves being different for sweeps in the positive or negative potential direction. The initial stages of oxidation of Cu(111) in nondeaerated 0.1 M NaOH solution were recently studied by cyclic voltammetry and the in situ STM technique in the underpotential range of Cu 2 O formation by Maurice et al. [6]. It was shown that the adsorption/desorption process occurs in this potential region (with a reversible potential of /0.6759/0.02 V vs. SHE), being initiated preferentially at the step edges on the upper terrace side inducing the lateral growth of terraces. The adlayer was found to form an ordered structure with a hexagonal lattice having a unit vector of 0.69/0.02 nm and two pffiffiffiffiffi p ffiffiffiffiffi pffiffiffiffiffi coincidence cells: ( )R10 and ( 49 p ffiffiffiffiffi 49 )R20 : Charge transfer measurements indicated adsorption of hydroxide or hydroxyl groups with one adsorbate per unit cell, i.e. a coverage of ca. 0.2 ML. The adlayer lattice parameters indicated that the reconstructed outermost Cu plane has the close-packing density and symmetry of the Cu planes in Cu 2 O(111), with the adlayer forming a (2/2) lattice of OH ads.it has also been stated in this paper that surface reconstruction for large values of the overpotential of adsorption or desorption was very fast (under the given conditions of the experiment). Most recently [7], the electrochemical behavior of Cu(111) and Cu(100) in 0.1 M NaOH has been investigated by cyclic voltammetry and the potentiostatic pulse technique. It was shown that slow and irreversible reconstruction of Cu(111) surface occurs as a consequence of the process of OH ion adsorption/ desorption. The same process on Cu(100) was found to take place at more negative potentials than on Cu(111) being characterized by fast j /t transients and two well defined voltammetric peaks, with no indication of surface reconstruction. In this paper the electrochemical behavior of Cu(111) and Cu(100) in 0.1 M NaOH has been investigated by cyclic voltammetry and EIS (differential capacitance) measurements. The approach presented in Part I of this paper [8] was used in order to determine the mechanism of OH ion adsorption and the influence of this process on the state of single crystal surfaces. 2. Experimental All experiments were performed in the same way as in Part I [8]. The reference electrode was a mercury/ mercury-oxide electrode (Hg/HgO in 0.1 M NaOH). The solution of 0.1 M NaOH was made from supra pure (99.999%) 50% NaOH solution (Fischer) and EASY pure UV water (Barnstead). Mechanical polishing of the copper single crystals was performed in the same way as in Part I [8]. After mechanical polishing, the electrodes were electrochemically polished in a solution of 85% phosphoric acid at a constant voltage of 1.7 V (vs. a Pt counter electrode) until the current density dropped to a value of about 10 ma cm 2. The electrodes were then washed thoroughly with pure water (Barnstead*/EASY pure UV), cleaned for 30 s in 10 vol.% H 2 SO 4, washed with pure water again and transferred into the electrochemical cell. 3. Results 3.1. Cyclic voltammetry Cyclic voltammograms recorded on the Cu(111) and Cu(100) faces at a sweep rate of 100 mv s 1 in 0.1 M NaOH solution, are shown in Fig. 1(a and b), respectively. After immersion of the electrodes into solution, the initial potential (negative with respect to the open circuit potential for about 0.3 V) was immediately applied and the electrodes were cycled in a given potential range starting towards the positive potential limit. As can be seen, the voltammogram of Cu(111), obtained after repetitive cycling between /1.15 V versus Hg/HgO and /0.50 V versus Hg/HgO for 30 min (full line) is characterized by one shoulder at about /0.90 V versus Hg/HgO and a sharp anodic peak at /0.82 V versus Hg/HgO and three cathodic peaks at about /0.85 V versus Hg/HgO, about /0.92 V versus Hg/

3 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 15 Fig. 1. Cyclic voltammograms of Cu(111) (a) and Cu(100) (b) in the solution of 0.1 M NaOH recorded at the sweep rate of 100 mv s 1. HgO and /1.06 V versus Hg/HgO, respectively. The shape of this voltammogram was found to depend on the time of holding the electrode at the initial potential and it changed slightly with number of cycles [7]. If the initial potential was set at /1.15 V versus Hg/HgO immediately after immersing the electrode in the solution, the first cycle was found to be completely different (dotted line) with no peak on the anodic part of the voltammogram and with an increase of anodic current density at more positive potentials (at about /0.6 V vs. Hg/HgO). The voltammogram obtained on Cu(100) was established after 2/3 cycles and it did not change with further cycling. As can be seen, this voltammogram is characterized by the presence of one pair of voltammetric peaks, an anodic peak at about /1.07 V versus Hg/HgO and a corresponding cathodic peak at about /1.10 V versus Hg/HgO. Hg/HgO), they were characterized by an almost complete semi-circle, while at more positive potentials only half of the semi-circle was obtained in the same range of frequencies. An example is shown for Cu(100) in Fig. 2 for the most negative potentials (a) and for the inter EIS and differential capacitance measurements Impedance measurements on both faces of copper were performed in steps of 50 mv starting at the most negative potential (/1.15 V vs. Hg/HgO) going towards more positive potentials (the most positive potential was the potential of the beginning of oxide formation) in the frequency range from 0.01 Hz to 30 khz. Before the impedance (differential capacitance) measurement, the electrodes were kept at the same potential for 5 min in order to establish a stable surface state. Z versus Zƒ diagrams recorded on both faces of a copper single crystal at different potentials were found to be very similar. At more negative potentials, approaching the negative potential limit (/1.15 V vs. Fig. 2. Z? vs. Z ƒ diagrams recorded in the frequency range from 0.01 Hz to 30 khz onto Cu(100) in 0.1 M NaOH at different potentials (marked in the figure in V vs. Hg/HgO). Squares, circles and triangles represent experimental points while the results of fitting are presented by lines. Fitting was performed by the equivalent circuit presented in Fig. 1(d) of Ref. [8].

4 16 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 mediate and most positive potentials (b), marked in the figure (V vs. Hg/HgO). The experimental results are presented by points (squares, circles and triangles), while lines represent the fitting curves. Fitting procedure was performed by using the equivalent circuit presented in the inset of Fig. 2(b). As can be seen in Fig. 2, good fits were obtained, but the values for C ad were extremely high indicating that this is not the best fit of the results obtained experimentally. C diff versus E curves for both faces of copper were recorded at the following frequencies: 1000, 700, 500, 300, 100, 70, 50, 30, 20, 10, 7, 5, 3, 2, 1, 0.7, 0.5, 0.3, 0.2, and 0.1 Hz. In Fig. 3 are shown curves recorded at 1000, 100, 10, 1 and 0.1 Hz. These curves were recorded by starting at the positive potential limit going (in steps of 10 mv) towards the negative potential limit. The reason for such an experimental procedure is the fact the Cu(111) undergoes slow and irreversible surface reconstruction [7] approaching a potential of about /0.70 V versus Hg/HgO. To avoid this effect, both electrodes were kept for 5 min at the positive potential limit before each run, so that Cu(111) could be considered completely reconstructed before each run. As can be seen, on both faces of the copper differential capacitance was found to decrease with increasing frequency, indicating the presence of a resistive component in parallel to the double layer capacitance and in series with the adsorption capacitance. It can also be seen that the shapes of the C diff versus E curves for both faces of copper are identical to the shapes of the anodic parts of the corresponding cyclic voltammograms shown in Fig. 1, except that the peak potentials on the C diff versus E curves are more negative than those of the CVs, which is the consequence of a sweep rate of 100 mv s 1 being used for recording the cyclic voltammograms. Since the voltammogram for Cu(100) was found to be very stable while cycling the electrode, C diff versus E curves (at 1 Hz) for this surface were recorded in both directions: from the positive to the negative potential limit and vice versa. The result of this experiment is shown in Fig. 5. As can be seen, although the voltammogram is very stable, a significant hysteresis in the potential region between /1.10 V versus Hg/HgO and /1.25 V versus Hg/HgO was detected on the C diff versus E curves, Fig. 4. C diff vs. E curves (corrected for R V ) recorded at the frequency of 1 Hz by changing the potential in steps of 10 mv in both directions for Cu(100): (j) starting at /1.25 V vs. Hg/HgO and finishing at / 0.80 V vs. Hg/HgO; (I) starting at /0.80 V vs. Hg/HgO and finishing at /1.25 V vs. Hg/HgO. Fig. 3. C diff vs. E curves (corrected for R V ) recorded on Cu(111) (a) and Cu(100) (b) at different frequencies starting from the positive potential limit and changing the potential towards the negative limit: (ji) 0.1; (mk) 1;('^) 10; (%\) 100; (.1) 1000 Hz.

5 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 17 while the value of C dl was obtained from Eq. (4) [8,9]. C dl [A dl R (a1) V ] 1=a (4) Fig. 5. C diff vs. v curves for different potentials obtained from C diff vs. E curves for Cu(111). Squares, circles, triangles, etc. represent experimental points, while full lines represent fitting curves obtained by using Eq. (3). Potentials in V vs. Hg/HgO are marked in the figure for each curve. indicating either mass-transport limitations or a change of the surface state. C diff versus v curves were obtained from C diff versus E curves at each potential. The values obtained experimentally are presented by circles, squares, triangles etc. while lines represent fitting curves. Fitting was performed by the equation for differential capacitance derived for the equivalent circuit presented in Fig. 1(d) of Ref. [8] C diff Yƒ ap v A C dl va1 sin ad (3) 2 1 v 2 Cad 2 R2 ct Good fits were obtained for all curves. Some of these curves for Cu(111) are presented in Fig. 5, while some of those for Cu(100) are shown in Fig. 6 (potentials in V vs. Hg/HgO are marked in these figures). As can be seen in Fig. 5, the curves recorded for Cu(111) possess identical shapes at all potentials. One inflection point at about v/10 Hz is clearly seen, while the other could not be detected for a given range of frequencies. In the case of Cu(100), the shapes of the C diff versus v curves are a function of the applied potential (Fig. 6). In a narrow range of potentials (between /1.20 V vs. Hg/ HgO and /1.17 V vs. Hg/HgO) the values of C diff are practically independent of frequency at frequencies lower than about v /100 Hz, indicating that a approaches unity (an ideal surface). At E ]//1.16 V versus Hg/HgO the shape of these curves is seen to change in the same range of frequencies. C diff is seen to increase with decreasing frequency, indicating a decrease of a (see Section 4). As in the case of Cu(111) only one inflection point has been detected on these curves too, but at a significantly higher frequency of about v/ 1000 Hz. By fitting the curves presented in Figs. 5 and 6 with Eq. (3), the values of A dl, C ad, R ad and a were determined at each potential. C dl (calculated using Eq. (4)), C ad,(c dl /C ad ) and a as a function of potential for Cu(111) are presented in Fig. 7(a), while C ad versus E and R ad versus E curves are shown in Fig. 7(b). As can be seen, the shapes of all the capacitance curves are identical (with C dl being higher than C ad ), while a is seen Fig. 6. C diff vs. v curves for different potentials obtained from C diff vs. E curves for Cu(100). Squares, circles, triangles, etc. represent experimental points, while full lines represent fitting curves obtained by using Eq. (3). Potentials in V vs. Hg/HgO are marked in the figure for each curve.

6 18 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 Fig. 7. (a) C ad vs. E (^), C dl vs. E (I), (C dl /C ad ) vs. E (k) and a vs. E (m) curves for Cu(111) obtained by fitting C diff vs. v curves for different potentials with Eq. (3), while C dl was obtained from Eq. (4); (b) C ad vs. E (I) and R ad vs. E (m) curves for Cu(111) obtained by fitting C diff vs. v curves for different potentials with Eq. (3). to change slightly with potential between 0.93 and As expected for such an equivalent circuit, the C ad versus E and R ad versus E curves change in opposite directions; when C ad increases, R ad decreases and vice versa. The same dependences for Cu(100) obtained by analysis of the curves recorded by changing the potential from the positive to negative potential limits are shown in Fig. 8. All capacitances are seen to be much higher than those for Cu(111). A significant change of a with potential between 0.39 and 1.00, the decrease of R ad is similar to the increase of C ad in the potential range between /1.20 V versus Hg/HgO and /1.10 V versus Hg/HgO, while at more positive potentials, the increase of R ad is much lower than the decrease of C ad. In Fig. 9 are presented a versus E (a), C ad versus E (b) and R ad versus E (c) curves for Cu(100) obtained by the analysis of C diff versus v curves recorded by changing Fig. 8. (a) C ad vs. E (^), C dl vs. E (I), (C dl /C ad ) vs. E (k) and a vs. E (m) curves for Cu(100) obtained by fitting C diff vs. v curves for different potentials with Eq. (3), while C dl was obtained from Eq. (4); (b) C ad vs. E (I) and R ad vs. E (m) curves for Cu(100) obtained by fitting C diff vs. v curves for different potentials with Eq. (3).

7 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 19 Fig. 9. (a) a vs. E, (b) C ad vs. E and (c) R ad vs. E curves for Cu(100) obtained by fitting C diff vs. v curves for different potentials with Eq. (3), recorded by changing the potential in both directions (marked with arrows in the figure). the potential in both directions (marked with arrows). As can be seen, these dependences are sensitive to the direction of the change of potential. 4. Discussion A systematic investigation of the electrochemistry of Cu(111) in 0.1 M NaOH solution in the underpotential range of Cu 2 O formation has practically been the subject of only three papers [5/7]. In these papers, the formation of adsorbed layer of OH species has been confirmed. Considering the voltammograms for Cu(111) and Cu(100) presented in Fig. 1, it is most likely that the process of OH species adsorption takes place on both faces of copper, being more complex on the Cu(111) surface. It has been shown recently for Cu(111) that such behavior during OH species adsorption is a consequence of the simultaneous lifting of the surface reconstruction [7]. Such behavior is in accordance with the findings of Maurice et al. [6] that adsorption of OH species modifies step edges corresponding to the lateral growth of terraces and also to the formation Cu islands on top of the terraces. It is interesting to note that the cyclic voltammogram obtained in an STM cell containing a non-deaerated solution [6] differs significantly from that obtained in our work in deaerated solution, indicating that most probably dissolved oxygen in the non-deaerated solution influences the processes of adsorption and desorption of OH species. It is quite difficult to explain the origin of the three cathodic peaks on the voltammograms presented in Fig. 1. The first and second cathodic peaks, at the potentials of about /0.85 V versus Hg/HgO and about /0.92 V versus Hg/HgO, respectively, most probably correspond to the desorption of adsorbed OH species, while the third peak at about /1.06 V versus Hg/HgO could be the consequence of reconstruction of the Cu(111) surface. It should be mentioned here that these cathodic peaks start to appear with the beginning of the reconstruction of the original Cu(111) surface [7]. Contrary to the electrochemical behavior of Cu(111) in 0.1 M NaOH solution, Cu(100) was found to be very stable, i.e. the voltammogram shown in Fig. 1 was obtained after a couple of cycles, it was insensitive to the initial potential value and the shape of the voltammogram did not change with number of cycles at any anodic limit potential lower than /0.6 V versus Hg/ HgO. From such behavior it could be concluded that Cu(100) either does not undergo surface reconstruction in this solution, or that the reconstruction is instantaneous and cannot be seen on the voltammograms. It should be mentioned here that the results of fitting the Z?/Zƒ diagrams (Fig. 2) and the results of fitting the C diff versus v curves (Figs. 5 and 6) gave different results for parameters C dl, C ad, R ad and a. This difference was more pronounced at more positive potentials where the Z?/Z ƒ diagrams were characterized by the half semi-circle (Fig. 2(b)), limiting the precision of fitting [8]. At the same time, significant dispersion of the points on these diagrams recorded at more negative potentials (Fig. 2(a)) at frequencies lower than 0.1 Hz also influenced the precision of fitting. As a result of this, extremely high values for C ad of the order of mf were obtained, not being in accordance with the value of the total capacitance corresponding to the

8 20 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 charge recorded under the voltammograms. Hence, the C diff versus v curves obtained in the frequency range from 0.1 to 1000 Hz were used for the analysis presented in this work. Considering the results presented in Figs. 3/8, itis obvious that the process of OH species adsorption/ desorption on both faces of copper involves an adsorption resistance R ad. First, the C diff versus E curves shown in Fig. 3 clearly indicate the presence of a resistive component in parallel with C dl and in series with C ad, since C diff decreases with increasing frequency. Second, the very good fit of the C diff versus v curves by Eq. (3) for both faces of copper presented in Figs. 5 and 6 also confirms that the adsorption resistance is involved in the process of OH species adsorption/desorption. Comparing the results presented in Fig. 7(b) and Fig. 8(b) for Cu(111) and Cu(100), respectively, one can see that the values of the adsorption capacitance (C ad ) for Cu(100) are much higher than those for Cu(111). Actually, by integration of the C ad versus E curves, the following amounts of charge were obtained: Q ad- Cu(111)/8 mc cm 2 and Q ad-cu(100) /47 mc cm 2.At the same time R ad values for Cu(111) were extremely high, varying between 10 4 and 10 5 V cm 2, while for Cu(100) these values were much smaller, being in the range between 1 and 5 V cm 2. Such behavior indicates that the process of OH species adsorption/desorption onto Cu(111) is much slower than that onto Cu(100) involving a much smaller amount (charge) of adsorbed OH species. Since it is confirmed that surface reconstruction is a consequence of OH species adsorption/ desorption onto the Cu(111) surface [6,7], such behavior is in accordance with our previous findings [7] that surface reconstruction, i.e. OH species adsorption/ desorption onto Cu(111) is a slow process. Comparing the charge under the (C dl /C ad ) versus E curves presented in Fig. 7(a) and Fig. 8(a) for Cu(111) and Cu(100), respectively, amounting to 46 mccm 2 for Cu(111) and 96 mc cm 2 for Cu(100), it can also be concluded that the total amount of OH species adsorbed is much higher for Cu(100) than for Cu(111). If one compares the charges obtained under (C dl /C ad ) versus E curves with the charges obtained by integration of the anodic parts of the corresponding cyclic voltammograms (presented in Fig. 1), one can see that there is significant discrepancy between these values for Cu(111) (Q CV /133 and Q (dlad) /46 mc cm 2 ), while the values for Cu(100) were very similar (Q CV /87 and Q (dlad) /96 mc cm 2 ). This indicates also that for fast adsorption processes, the charge under the cyclic voltammograms can be used for determining the total amount of adsorbed species, while in the case of a slow adsorption process, the amount of charge obtained by integration of the voltammogram could lead to a wrong conclusion. It appears that by the analysis presented in this work it is possible to determine the charge involved in the formation of the double layer (purely capacitive charge) and the charge corresponding to the process of anion adsorption. An interesting feature of the results presented in this work is the dependence of a as a function of potential, indicating the change of the surface state. Such behavior is first indicated by the results presented in Fig. 4 for Cu(100). Since at a concentration of 0.1 M, NaOH mass-transport of OH species cannot influence the process of their adsorption [8,10/17] (particularly with the values of R ad in the range between 1 and 10 V cm 2, Fig. 9(c)), the hysteresis of the C diff versus E curve can only be the consequence of a change of surface state. In the case of Cu(111), a changes between 0.93 and 0.96 indicating that adsorption (surface reconstruction) does not change the state of the surface significantly under the condition that the surface has previously been exposed (for 5 min) to potentials equal or more positive than /0.7 V versus Hg/HgO, where surface reconstruction begins [7], i.e. these values of a were obtained for an already reconstructed Cu(111) surface. For Cu(100) this change is seen to be much more pronounced (Fig. 8(a)) with the shape of the a versus E curve being sensitive to the direction of the changing potential, as can be seen in Fig. 9(a). If the potential is changed from the positive to negative potential limit, the value of a changes from 0.6 to 1.0 with a minimum of about 0.4 at the potential of about /1.07 V versus Hg/HgO. In the potential region of negligible adsorption of OH species (E 5//1.15 V vs. Hg/HgO), the electrode surface retained its original value of a/1.0 (Fig. 9(a)*/dotted line). If the potential is changed from the negative to positive potential limit, a is seen to change from 0.98 to 0.73 with a minimum of about 0.70 at the same potential as in the previous case (about /1.07 V vs. Hg/HgO). Taking into account that a represents the fractal character of the surface, i.e. the capacitance dispersion is due to adsorption effects [10 / 17] and that reconstruction of the surface is a consequence of OH species adsorption/desorption [6,7], then, for a more pronounced change of a, a greater amount of OH species adsorbed should be expected. This is exactly what is happening on Cu(100), since the C ad versus E curves presented in Fig. 9(b) clearly confirm this statement. Hence, although from the cyclic voltammetry results it seems that Cu(100) is more stable, not undergoing surface reconstruction, the results of the analysis of differential capacitance show significant and very fast reconstruction of the original Cu(100) surface during the process of OH species adsorption/desorption. Taking into account that surface reconstruction, as a consequence of OH species adsorption/desorption, has been detected on Cu(111) [6,7] and that this reconstruction is slow [7], it seems reasonable to expect that a similar process should take place on Cu(100), and be more pronounced, since the original

9 V.D. Jović, B.M. Jović / Journal of Electroanalytical Chemistry 541 (2003) 13/21 21 surface structure of Cu(100) is more open than that of Cu(111). It is interesting to note that in the potential range of low values of a (E ]/1.10 V vs. Hg/HgO) for Cu(100), the R ad versus E curves (Fig. 8(b) and Fig. 9(c)) behave somewhat differently than expected for a series connection of R ad and C ad. Instead of a sharp increase of R ad following a sharp decrease of C ad (the opposite change in comparison with the change of C ad ), R ad is seen to increase slightly, indicating that a change of the surface state influences the values of R ad. If that were just a surface roughness effect, then it would have the same influence on the value of C ad. Since this is not the case, it seems most likely that the fractal character of the surface does not represent just the roughness of the surface, but is connected with the charge distribution on the surface. This finding is in accordance with a recent statement of Kerner and Pajkossy [16] concerning the frequency dependence of the capacitance on solid electrodes and the introduction of a CPE instead of C dl. Finally it should be emphasized here that the approach presented in this work provides more information about the process OH species adsorption/ desorption and surface reconstruction on Cu(111) and Cu(100) in 0.1 M NaOH than cyclic voltammetry and potentiostatic pulse results [7]. 5. Conclusion By the analysis of C diff versus E and C diff versus v curves for Cu(111) and Cu(100) in 0.1 M NaOH, it is shown that the process of OH species adsorption/ desorption taking place in the underpotential region of Cu 2 O formation on both faces of copper, can be described by the adsorption capacitance and resistance connected in series. It is also shown that on Cu(111), slow reconstruction of the original surface occurs during the process of OH species adsorption/desorption, while on Cu(100), this process was found to be very fast involving a much greater amount of adsorbed OH species. References [1] J.M.M. Droog, C.A. Alderliesten, P.T. Alderliesten, G.A. Bootsma, J. Electroanal. Chem. 111 (1980) 61. [2] J. Ambrose, R.G. Barradas, D.W. Shoesmith, J. Electroanal. Chem. 47 (1973) 47. [3] J.M.M. Droog, B. Schlenter, J. Electroanal. Chem. 112 (1980) 387. [4] S. Härtinger, K. Doblhofer, J. Electroanal. Chem. 380 (1995) 185. [5] S. Härtinger, B. Pettinger, K. Doblhofer, J. Electroanal. Chem. 397 (1995) 335. [6] V. Maurice, H.-H. Strehblow, P. Marcus, Surf. Sci. 458 (2000) 185. [7] V.D. Jović, B.M. Jović, J. Serb. Chem. Soc. 67 (2002) 531. [8] V.D. Jović, B.M. Jović, J. Electroanal. Chem. 541 (2003) 1. [9] G.J. Brug, A.L.G. van Eeden, M. Sluyters-Rehbach, J. Sluyters, J. Electroanal. Chem. 176 (1984) 275. [10] V.D. Jović, B.M. Jović, R. Parsons, J. Electroanal. Chem. 290 (1990) 257. [11] V.D. Jović, R. Parsons, B.M. Jović, J. Electroanal. Chem. 339 (1992) 327. [12] B.M. Jović, V.D. Jović, D.M. Drazić, J. Electroanal. Chem. 399 (1995) 197. [13] B.M. Jović, D.M. Drazić, V.D. Jović, J. Serb. Chem. Soc. 61 (1996) [14] D. Eberhardt, E. Santos, W. Schmickler, J. Electroanal. Chem. 419 (1996) 23. [15] T. Pajkossy, T. Wandlowski, D.M. Kolb, J. Electroanal. Chem. 414 (1996) 209. [16] Z. Kerner, T. Pajkossy, Electrochim. Acta 46 (2000) 207. [17] T. Pajkossy, J. Electroanal. Chem. 364 (1994) 111.